Monocrystalline silicon wafers of 200 mm (8") diameter are becoming available in sufficient quality and quantity that semiconductor manufacturers will be using this size (and perhaps eventually larger) wafers for device fabrication. One technique for doping silicon wafers is to direct a beam of ions to impinge upon a target wafer to produce controlled concentrations of doping impurities within the wafer. Demand for ion implanters capable of successfully implanting these larger wafers should build as the wafers become available.
A critical parameter of semiconductor ion implant processes is the spatial uniformity of the implanted dose across the wafer. Another critical parameter is the angle of incidence of the ion beam with respect to the wafer and its internal silicon crystal lattice structure (or GaAs lattice or other crystalline substrate). The angle of incidence is important because of the role it plays in the well known phenomenon of channeling which changes the dopant depth profile when incident angle varies as a function of position on the wafer surface. Incidence angle variation also can produce shadowing effects when the implant is performed through a mask to control implanted areas.
As VLSI technology requires smaller and smaller integrated device sizes, implant mask openings get smaller and smaller while mask thickness does not decrease as rapidly. Thus, shadowing effects are becoming more and more important as device performance specifications are becoming more demanding and depth nonuniformities that are spatially distributed across wafers due to channeling variations are also becoming less tolerable.
In medium and low current ion implanters the ion beam has commonly been directed across the wafer surface by x-y deflection scanning of the beam in a raster or similar pattern across the wafer. This has commonly been done using two orthogonal pairs of electrostatic deflection plates to produce the beam deflections. Application of triangular waveform voltage to the plates can produce rectangular raster scan patterns on the wafer. U.S. Pat. No. 4,514,637 to Dykstra et al. discloses one such medium to low current ion implanter. The disclosure of Dykstra et al. patent is incorporated herein by reference.
Electrostatic ion beam deflection as described above results in different angles of beam incidence at different locations on the wafer surface. This is the source of a major depth nonuniformity (from the resulting channeling variations) which occurs in this type of implanter.
Due to the difficulty of transporting heavy high current ion beams long distances, especially in the presence of electrostatic fields, this type of beam scanning to spread the beam uniformly over large wafers has not been practical in high current implanters. High current implanters have tended to scan the wafer past a stationary beam by mechanical means such as attaching the wafers to a spinning disk passing through the beam or by other well known mechanical scanning techniques. In recent years these mechanical scanners have tended to be designed so that as the wafer moves through the ion beam, the angle of beam incidence remains constant or nearly so. For this reason mechanical scanning of wafers through a stationary beam has come to be considered as a superior method to x-y deflection of the beam for producing depth uniformity and minimum mask shadows variation.
The batch handling imposed by the high beam density mechanical scanning techniques have generally resulted in reduced wafer handling throughput and large costly wafer handling stations. X-Y deflection scanning machines on the other hand have advantages in size, simplicity, and cost but suffer from varying angle of beam incidence problems. As wafer diameters become larger the angle of incidence increases unless the length of the beam transport is proportionally increased. A lengthening of the beam path, however, would result in severe problems in beam transport resulting in low delivered beam currents on the target wafer.
Another method of addressing the angle of beam incidence problem has been the use of parallel scanning techniques. In an electrostatic scanning system, this can be accomplished by using 2 sets of deflection plates, each consisting of 2 orthogonal pairs of plates. The first set is used to deflect the beam in a raster or similar pattern, as described above. The second set of plates is then used to deflect the beam back in a direction parallel to the original beam direction. The result of this is a raster scanned beam that has a very nearly constant angle of incidence at all points on the wafer. This technique requires that the second set of plates be spaced at least as far apart as the size of the target. Since the required deflecting voltage is directly proportional to the plate spacing, this technique is generally limited to small wafers. Further, because of the problem of transporting ion beams in electrostatic fields, as previously noted, this technique is also limited to use with low mass and low current ion beams.
Recent experiments have indicated that the effects of varying dopant depth profiles caused by channeling as the angle of incidence changes across the wafer is the most severe contribution to nonuniform sheet resistivity even on small wafers having 3" and 4" diameters for a properly designed x-y system.